The present invention relates to a battery pack including, for example application, a lithium-ion secondary cell.
A secondary cell, for example, a lithium-ion secondary cell is compact size and light weight, and has a high energy density, high outputs, or the like. Accordingly, a secondary cell is widely used as a power source of application equipments such as a digital camera, a mobile phone, a portable PC, and a camcorder (abbreviation of camera and recorder). The lithium-ion secondary cell is used by repeating discharging and charging, but if the cell is left in the full charged state or at nearly full charged state, cell performance tends to be lowered and life of the cell tends to be shortened. Especially, left conditions, such as under high temperature and for long hours, conspires to make the degradation of the secondary cell prominent.
For example, if charging and discharging are repeated in a manner that first, lithium-ion secondary cells are charged at 4.1V or 4.2V respectively, and then left for predetermined hours under the high voltage state, and thereafter discharged down to 3.0V, the discharge capacity of one lithium-ion secondary cell left at 4.1V is not so lowered even if 300 cycles are exceeded. In contrast to this, the discharge capacity of the other lithium-ion secondary cell left at 4.2V begins to decrease when 200 cycles are exceeded, and the discharge capacity is decreased abruptly when 250 cycles are exceeded. Thus, if a battery pack having a secondary cell, especially a lithium-ion secondary cell, is left for long hours in a high cell voltage state, degradation speed of a cell may be fast.
In view of the features, a technology for performing charging by changing full charge voltage of a cell by a switch, and selectively using a high capacity mode of high set voltage and a long-life mode of set voltage lower compared to that of the high capacity mode, is disclosed in Japanese Unexamined Patent Application Publication No. 2002-78222 (hereinafter referred to as “Patent Document 1”).
Further, Japanese Unexamined Patent Application Publication No. 2003-125540 (hereinafter referred to as “Patent Document 2”) discloses that the protection function control means of a battery pack monitors the terminal voltage of a secondary cell, and when overcharging is detected, warning is displayed to flow self-consumption current, whereby the voltage of the secondary cell is lowered to the safe voltage area in an early stage.
However, as to the technology disclosed in the Patent Document 1, a user is required to set either the high capacity mode or the long-life mode before charging a secondary cell. Therefore, once the cell is charged in the high capacity mode and left at high voltage state, there is no means for lowering the voltage. Further, uneasy to handle situations, such as a case where a user forgets to switch due to carelessness, are conceivable.
Also, equipments, such as a digital camera and a camcorder, which are not always carried and used by a user, tends to be unused for a relatively long time from when the equipments are used to subsequent time the equipments are used. For example, general users have a few opportunities to use camcorders, or the like everyday. On the other hand, when these equipments are used, a secondary cell tends to be charged at night, which is one day before the equipments are used, or even in the case of travel abroad, mountain climbing, and the like, the equipments tend to be used within one week after the cells are charged. Thus, while charging and discharging of the cells are repeated at relatively short time periods of cycles, the cells of the equipments tend to be left in an unused state for a long time. During the unused time periods, voltages of the secondary cells are left at substantially same state with voltages of last use. Although voltage state of a secondary cell during unused time varies depending on a use method by a user, if cells are left at high voltage state after last use, it is probable that the cells are left at high voltage state for a long time, such as several months or one year, which results in further deterioration of the secondary cells.
Further, in a battery pack, an overcharge detection voltage value in which a protection circuit functions as overcharge protection is set to be higher than a full charge voltage value of a secondary cell by around 0.1V per a single secondary cell (hereinafter referred to as “single cell”). Therefore, even if a voltage value exceeds the full charge voltage value, a protection circuit does not function until the voltage value becomes equal to or larger than the overcharge detection voltage value. Thus, the voltage of a secondary cell may be charged up to a voltage value in a range from equal to or larger than full charge voltage to equal to or less than overcharge detection voltage value. Furthermore, a secondary cell is further deteriorated if the cell is left at such high voltage state. The battery pack described in the Patent Document 2 may not solve an issue that a cell is charged up to a relatively high voltage and left at the high voltage state.